Transmissive liquid crystal display (LCD) is widely used as information display tools, such as cell phone, personal digital assistant, laptop computer and so on. The most commonly used transmissive twisted-nematic (TN) LCD has a 90° TN liquid crystal layer sandwiched between two perpendicularly rubbed transparent substrates with Indium-Tin-Oxide (ITO), coatings. Two linear polarizers are placed at the outside of transparent substrates to act as a polarizer and an analyzer whose transmission directions are either parallel or perpendicular to the rubbing direction of the adjacent substrate. In addition, a backlight is put outside of the polarizer as the light source. Without voltage, the incident light becomes linearly polarized after passing through the polarizer, then follows the twist structure of TN liquid crystal layer, and finally transmits through the analyzer, resulting in a bright state. When the applied voltage exceeds the threshold voltage, the twist structure of TN liquid crystal layer is broken and the incident linear polarizer can not follow the liquid crystal twist structure; consequently, the light, in general, becomes elliptically polarized and the output transmittance decreases. If the applied voltage is high enough, the volume part of the liquid crystal molecules are approximately aligned perpendicularly to the substrates, except the crossed residual boundary liquid crystal layers. In this case, the incident linearly polarized light nearly maintains the same polarization state after passing through the entire liquid crystal layer, and then is blocked by the analyzer, resulting in a very good dark state. A major drawback of the transmissive LCD is that its backlight source should be on all the time when the display is in use; therefore, the power consumption is relatively high. Another disadvantage is that the image of transmissive LCD is easily washed out under strong ambient light conditions, such as outdoor sunlight.
Reflective LCD, on the other hand, has no built-in backlight source. Instead, it utilizes ambient light for reading the displayed images. U.S. Pat. No. 5,933,207 issued to Wu on Aug. 3, 1999 describes a reflective LCD comprising a polarizer, a phase compensation film, a liquid crystal layer, and a reflector. Compared to the transmissive LCD, the reflective LCD has advantages including low power consumption, light weight, and good outdoor readability. However, a reflective LCD relies on ambient light and thus is inapplicable under low light levels or dark ambient conditions.
To utilize the advantages, and overcome the disadvantages, of both transmissive LCD and reflective LCD, the transflective LCD is used in the apparatus, method, system, and device of the present invention. Transflective LCD means the apparatus displays an image in transmissive display mode and reflective display mode either independently or simultaneously. Therefore, such a transflective LCD is designed to be used under any ambient circumstances; U.S. Pat. No. 4,315,258 issued to McKnight et al on Feb. 9, 1982 proposed a transflective LCD design shown as 10 in FIG. 1. It consists of a front polarizer 11, a LC panel 12, a rear polarizer 13, a transflector (partially transmitting mirror) 14 and a backlight source 15. Such a structure is actually modified from the conventional transmissive twisted-nematic (TN) LCD by putting a transflector 14 between the rear polarizer 13 and backlight source 15. This prior art has the advantages of a simple manufacturing process and low cost; however, it suffers from serious parallax problem because the ambient light passes through a very thick glass substrate before it hits the transflector. When the display device is viewed from an oblique direction, the reflected beam and input beam pass through different pixel areas, resulting in a shadowed image, which is called parallax. Such a parallax problem becomes increasingly serious when the pixel size decreases in high resolution display devices.
To overcome the parallax problem, the transflector should be imbedded in the inner side of the bottom substrate. U.S. Pat. No. 6,281,952 to Okamoto et al proposed a transflective LCD design shown as 200 in FIG. 2. It consists of a top linear polarizer 201a and a bottom linear polarizer 201b, a top compensation film 202a and a bottom compensation film 202b, a top transparent substrate 203a and a bottom transparent substrate 203b, a liquid crystal layer 208 sandwiched between the top substrate 203a and the bottom substrate 203b. The top substrate 203a is coated with a transparent electrode 204a and a first alignment film 205a. The bottom substrate 203b is coated with a transflector means 212, which contains a non-uniform thickness isolation layer 206, a transparent electrode 204b and a patterned reflection layer 207. The reflection layer 207 only covers the thick isolation layer region, which defines the reflective display region 210. The thin isolation layer region, which defines the transmissive region 211, is not covered with the reflection layer 207. Above the transflector means 212 is a second alignment film 205b. The liquid crystal layer 208 contacts with both the first alignment film 205a and the second alignment film 205b. A backlight source 209 is provided outside of the bottom polarizer 201b to function as the light source for the transmissive display region 211. Since the transflector means 212 was deposited inside of the bottom substrate 203b, the reflected beam does not pass through the very thick bottom substrate 203b; therefore, the parallax problem is eliminated. In addition, in order to compensate the optical path difference between the reflective and transmissive display modes, the cell gap in transmissive display region 211 is thicker than that in reflective display region 210 or the director alignment mechanism in transmissive display region 211 is different from that in reflective display region 210. Nevertheless, in either case, the manufacturing process is quite complicated and hence the manufacturing cost is relatively high. Another drawback of the different cell gap approach is that the response time in reflective region 210 is different from that in transmissive region 211 since the response time is proportional to the square of cell gap. Furthermore, the different cell gap or different alignment for transmissive and reflective display regions will introduce a disclination line on the border of two regions, which leads to dark state light leakage and thus degraded contrast ratio of the displayed image.
To solve the cell gap difference problem while keeping parallax-free in transflective CD, US patent application No. 20030202139 by Choi et al disclosed a transflective LCD design with partial switching method shown as 300 in FIG. 3. It consists of a top substrate 301a coated with a top transparent electrode 302 and an alignment film 303a, a bottom substrate 301b coated with a transflector means 304 and an alignment film 303b, and an liquid crystal layer 305 sandwiched between the top substrate 301a and bottom substrate 301b. The transflector means 304 is composed of a non-patterned (continuous) transparent electrode 304a, a patterned (discontinuous) transparent electrode 304b, a reflector 304c below the patterned transparent electrode 304b, and an insulating layer 304d. The non-patterned transparent electrode 304a area defines the transmissive display region 306, while the reflector 304c area defines the reflective display region 307. The non-patterned transparent electrode 304a and the patterned transparent electrode 304b are connected with each other and they have the same electric potential. The electric field between top transparent electrode 302 and bottom non-patterned transparent electrode 304a is strong and almost perpendicular to the substrates 301a and 301b. Such a strong electric field drives the liquid crystal molecules 305a to almost fully tilted as shown in FIG. 3. While the electric field between top transparent electrode 302 and bottom patterned transparent electrode 304b is a fringing field and its overall strength is weaker than the field above the non-patterned transparent electrode 304a. Such a weak fringing field only drives the liquid crystal molecules 305b partially tilted.
Therefore, the phase retardation in reflective region is approximately half of that in transmissive region. However, since the reflector 304c should be located under the discontinuous electrode 304b, the insulating layer 304d is inevitable, which increase the manufacturing process. To avoid use of an insulating layer 304d, the discontinuous electrode 304b can be coated on the top substrate 301a. In either case, however, the weak electrical field only exists between the discontinuous electrode gap and the common electrode 302, while the electrical field right above the discontinuous electrode 304b is still as strong as that in transmissive region 306. In other words, not the whole reflective display region is governed by fringing field. Consequently, the local region liquid crystal molecules above the discontinuous electrode 304b are still full-tilted as in transmissive region 306. Therefore, the gray scale of reflective and transmissive display modes still does not overlap very well, as shown in FIG. 6 of US patent application 20030202139.